1. Field of the Invention
The present invention relates to control devices for repetitive limit switching in connection with a lineally reciprocating device, and, more particularly, to a displacement transducer for such a reciprocating device which utilizes a rotary potentiometer to produce a switching signal value of high accuracy, as required in connection with injection molding machines, for example.
2. Description of the Prior Art
The present invention is related to a device disclosed in my U.S. Pat. No. 4,290,317 which features a precision drive for a rotary potentiometer to measure the lineal displacements of a reciprocating machine unit, such as the injection movement of the plastification screw of the injection unit in an injection molding machine. It is very important for this displacement-transducing action to be of utmost precision and consistency, because the resultant switching signal determines the quantity of plastic material which is injected into the injection molding die of the machine, thus affecting the quality of the injection-molded parts.
In my prior disclosure, I have suggested the arrangement of a timing belt loop alongside the injection unit in such a way that the relative axial displacement between the plastification cylinder and the plastification screw is transmitted to a parallel-oriented run of the timing belt. The lineal movement of the latter is thereby translated into an angular movement of the belt pulleys, one of which serves as a control disc, in a direct drive connection with a rotary potentiometer. Accordingly, the angular position of the potentiometer wiper and the resulting signal output are accurate indicators of the displacement position of the plastification screw. A screw-type injection unit normally has two parallel tie rods which carry a first bridge assembly with a central plastification cylinder on the forward side of the unit and a second bridge assembly with the plastification screw and a hydraulic screw drive on the rear side of the unit, one or two hydraulic cylinders producing the injection stroke by approaching the two bridge assemblies.
It has now been found that, while it is necessary to directly couple the control disc of the timing belt loop to the rotary potentiometer for a backlash-free angular connection, a rigid connection between the potentiometer drive shaft and the hub of the control disc can produce undesirable side effects in the bearings of the potentiometer drive shaft, if the potentiometer is mounted in a normal, fixed arrangement on the supporting structure. The potential problems include a reduced longevity of the potentiometer, with less than optimal measurement accuracy, and assembly problems that lead to further inaccuracies, if the potentiometer is removed and reinstalled, for any reason.